Remove appveyor configuration (#23251)
Remove appveyor configuration (#23251)

We're no longer using Appveyor.


tvolkert/flutter,flutter/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,tvolkert/flutter,flutter/flutter,flutter/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter